In present day practice current measurement on multi-kilovolt transmission lines is performed using current transformers, with the lines carry polyphase currents acting as primary windings, and with secondary windings operated near ground potential. The windings of the current transformers have to be electrically isolated with electrical insulation that will withstand the multi-kilovolt potential between them. The multiple-core transformer used for polyphase line voltage and current monitoring costs several thousand U.S. dollars, installed. The continuing measurement of active power in a set of power transmission lines is of concern to avoid overloading them. It is customary to use four current transformers delivering secondary currents up to 5A and three voltage transformers delivering secondary voltages up to 110 or 220 in each phase of a three-phase high-tension (HT) transmission line. The secondary winding of one of the current transformers is used in power measurement and the secondary windings of the others are loaded with relays used in protection against undesirable over-current, ground-fault, line-fault, distance, over-voltage, directionality, under-current and under-voltage conditions.
Garbuny in U.S. Pat. No. 3,312,895 issued April 4, 1967 and entitled "Transmission Line Monitor Apparatus Utilizing Electromagnetic Radiation between Line and a Remote Point" has described coupling to an infrared radiating device a portion of the power which is radiated from an HT power transmission line into a surrounding electro-magnetic field. The radiation from the infrared-radiating device is then remotely sensed with an infrared detector cell to generate a signal indicative of the current passing through the transmission line. This scheme is unattractive since an infrared-radiating device and its associated parabolic reflector must be installed on the multi-kilovolt transmission line which introduces problems with power system reliability. The installation then has to be maintained without having to shut down transmission. The installation also precludes portability in the measuring equipment.
Further, modern HT transmission lines are typically constructed with a steel supporting core wound around with helices of aluminum conductor, and these helices are counterwound to diminish substantially the surrounding electromagnetic field that undesirably cause conduction only near the conductor surface. This lessens the power available for the Garbuny device. This construction desirably reduces the skin effect which causes the resistance of the conductor to vary as a function of current and to exhibit a departure from linear change in resistance with increasing temperature. The bulk resistance of the conductor becomes substantially the entire resistance, or R, term in determining the I.sup.2 R loss along a length of the line; and the linear increase in the bulk resistance with increased temperature facilitates the measurement of current in the conductor by proceeding from the temperature of the conductor itself, as is done in the present invention.